Population size, distribution and habitat selection of the white-tailed eagle Haliaeetus albicilla in the alluvial wetlands of Croatia

From 2003-2006, research on the breeding distribution of the white-tailed eagle (Haliaeetus albicilla) was conducted in Croatia in order to assess the size of the national population. In 125 locations, clear signs of breeding activity were found. An additional 10 presumably active territories were detected but it was not possible to locate the exact position of the nests and confirm the breeding. Based on this, it is concluded that the national breeding population is not less than 135 breeding pairs. The present distribution can be compared with previous reports with the exception of the area along the Ilova and Lonja rivers that have never been reported as an important breeding site. Analysis of the characteristics of 138 nest positions as well as preferences/avoidance of specific structural features were performed. The results showed that white-tailed eagles prefer to build their nests on pedunculate oaks, narrow-leafed ash and white poplars with the greatest preference for mature trees with a diameter above 92.5 cm. The minimal distance between two active pairs was 348 meters. More than 50% of the national population breed less than two km from a large water area and 95% of the population less than four km. More than 95% of the population breed at altitudes lower than 140 m above sea level and are further than one km away from the nearest human settlement, regardless of the availability of forests. According to several parameters (distance to a large water area, elevation, forest presence, distance to the nearest settlement, distance to highways and railways) geographic information system (GIS) helped to determine potential white-tailed eagle breeding areas.